Move OpenSSL initialization to a single place, make library usage thread-safe.

While doing some reading about OpenSSL, I noticed a couple of things that
needed to be improved with our usage of OpenSSL.

1) We had initialization of the library done in multiple modules.  This has now
   been moved to a core function that gets executed during Asterisk startup.
   We already link OpenSSL into the core for TCP/TLS functionality, so this
   was the most logical place to do it.

2) OpenSSL is not thread-safe by default.  However, making it thread safe is
   very easy.  We just have to provide a couple of callbacks.  One callback
   returns a thread ID.  The other handles locking.  For more information,
   start with the "Is OpenSSL thread-safe?" question on the FAQ page of
   openssl.org.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@205120 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/_private.h

@@ -44,6 +44,7 @@ int ast_indications_reload(void);/*!< Provided by indications.c */
 void ast_stun_init(void);               /*!< Provided by stun.c */
 int ast_cel_engine_init(void);		/*!< Provided by cel.c */
 int ast_cel_engine_reload(void);	/*!< Provided by cel.c */
+int ast_ssl_init(void);                 /*!< Porvided by ssl.c */
 
 /*!
  * \brief Reload asterisk modules.